Command to generate c = "a + b" in console, when 1000's of value "a" and "b" added in MySQL manually

I have three columns in my database:

a = value b = value c = a + b

User usually enters a and b in the form, and the value is stored in the database. c will be calculated by the model.

In my model:

class Calculate < ActiveRecord::Base
  before_validation_on_create :update_c
  before_validation_on_update :update_c

  def update_c
    if !a.blank? && !b.blank?
      self.c = a + b
    end
  end
end

Just a sample code above. So c is generated everytime the user clicks "Save".

Now, I am an admin, and I populated 1000's of values for a and b using phpMyAdmin CSV import:

id=1, a=4, b=7, c=
id=2, a=1, b=6, c=
id=3, a=3, b=2, c=
.
.
.

Milliondollar question:

What is the best way to do it in console or somewhere else, to tell Rails to generate C for all the 1000's of entries?

Thank you!

UPDATE 1:

Actually the c = a + b is not that simple in my code. Take geocode_address as c, address_geo as a, b, here is the full code:

def geocode_address
if !address_geo.blank?
  geo=Geokit::Geocoders::MultiGeocoder.geocode(address_geo)
  errors.add(:address开发者_开发知识库, "Could not Geocode address") if !geo.success
  self.lat, self.lng = geo.lat,geo.lng if geo.success
end

end

I would just connect to MySQL directly (or run the SQL through phpMyAdmin) to issue an update statement.

I'm assuming you want to update ALL the records.

UPDATE table SET c = a + b;

That'll do it for you, nice and fast. I'm guessing you know what the table name is, I'm not familiar with Rails. I don't think you need Rails for this one.

Try this:

Calculate.update_all("c = a+b")